1. Field of the Invention
The present invention relates to a dockleveller installation. More particularly, it relates to a dockleveller installation having a hydraulic cylinder, for pivoting the dockboard of the installation between cross-traffic and bridging positions, and to the system for actuating the cylinder.
2. Description of the Prior Art
A dockleveller installation is a device which provides a bridge to span the gap between the surface of a loading dock and the bed of a truck backed up to the dock. The installation includes a dockboard, which is a flat, heavily reinforced platform. This dockboard is hinged at its inner or rear edge to the frame of the installation. Thus it can be pivoted between a horizontal cross-traffic position (in which it forms part of the loading dock surface) and an inclined bridging position.
Means must be provided to pivot or raise the heavy dockboard from the cross-traffic to the bridging position. In most cases such means comprise a hydraulic cylinder, a pneumatic cylinder, or a spring. The present invention is concerned with the cylinder-biased type of installation.
Each of the prior art pneumatic and hydraulic pivoting systems are characterized by both advantages and disadvantages.
Let us consider some of the disadvantages of the hydraulic pivoting system. This system normally comprises an electric motor, a hydraulic pump driven by the motor, a fluid reservoir, and suitable lines and connections completing the system. If the facility (such as a warehouse) requires a number of docklevellers, then a separate motor and pump must be provided for each unit. This is expensive. Also, many working parts are involved, so the likelihood of mechanical failure is high. Another disadvantage of this system is that the reservoir is usually vented to the atmosphere in the facility. This is undesirable in a food-processing plant, as the reservoir emits polluting compounds. Still another disadvantage is that moisture may enter the system through the open reservoir; this can be troublesome if the lines are exposed to freezing temperatures. If the system is to be installed in a harardous area, it is usually necessary to locate the motor and pump outside this area, unless an expensive explosion-proof unit is used. This results in having to provide a long hydraulic line extending from the pump to the dockleveller; there is thus a high pressure drop across this line and the output pressure at the cylinder is correspondingly diminished. Still another disadvantage has to do with the capability of an electric-hydraulic system to build up a high output pressure at the cylinder. If a forklift truck has been left parked partly on the dockboard, and the cylinder is activated, the pressure generated can be sufficient to overturn the truck.
A hydraulic pivoting system is, however, characterized by some positive aspects as well. For example, it may be accurately monitored, controlled and adjusted with respect to the output delivered by the cylinder. This is so because the amount of fluid moving through the line can be closely controlled. Also, it is a reliable system when exposed to freezing temperatures (although, as previously pointed out, problems can arise from the entry of moisture into the lines through the open reservoir).
Use of a pneumatic pivoting system does away with many of the shortcomings of the hydraulic system. For example, a single air compressor can be used to power all the cylinders of a multiple dockleveller facility. Thus lower capital and maintenance costs can be obtained by use of the pneumatic system. In many buildings, an air compressor is already in place and part of its output can be used to power the new dockleveller cylinders. Also, there is no reservoir associated with a pneumatic system, so atmospheric pollution is not a problem. Furthermore, pressure drops due to friction are much reduced in a pneumatic system, as compared to a hydraulic system. Thus long supply lines can be used without disadvantage. In addition, an air system has a built-in accumulator capability which assists in producing a constant and safe pressure output at the cylinder.
Now, while the pneumatic system does have a number of advantages relative to the hydraulic system, it has some disadvantages which have prevented it from achieving commercial success. More particularly, in cold climates customers fear condensation and freeze up in exposed lines. Also, the system does not lend itself to close control and adjustment of the output pressure at the cylinder.
With this background in mind, it is the object of this invention to provide a dockleveller installation, having a cylinder actuating system, which incorporates many of the best features of both the hydraulic and pneumatic systems and which does not have their disadvantages.